Gas chromatography is a method of separating substance mixtures. A sample of the substance mixture is vaporized. The vapor is “flushed”, by means of an inert carrier gas flow, through a separating column filled with a separating substance. The various substances or components of the substance mixture will migrate through the separating column at different velocities, depending on their affinity to the separating substance such as the solubility in a liquid separating substance. The components of the sample fed to the inlet of the column together as a “plug”, will appear at different times consecutively at the outlet of the column and will be detected by a detector responding to such components. When such component emerges from the column, the detector provides a “peak”, i.e. for example, a tooth or bell shaped signal.
So-called “capillary” columns are a narrow tube the inner wall of which is coated by a usually liquid separating substance. In contrast thereto, so-called “packed” columns comprise a relatively wide tube which is filled with a granular separating substance or of granulates coated with a separating substance. Capillary columns provide high resolution, i.e. good separation of the various peaks. Capillary columns, however, can process small sample quantities only.
In order to be able to use sample quantities which can be handled easily, the sample is usually dissolved in an easily volatile solvent. This solvent should be volatilized and removed prior to the entering of the sample into the capillary column.
It is known (G. Schomburg, “Probenaufgabe in der Kapillargaschromatographie” in “LABO—Kennziffer-Fachzeitschrift für Labortechnik” July 1983) to dispense a liquid sample into a pre-separator in the form of a vaporizer tube filled with quartz wool. This vaporizer tube is surrounded by a programmable heater. The inlet end of the capillary column extends into the outlet end of the vaporizer tube.
Vaporizer tubes (“liners”) are known, which have one or two spaced throats or reduced diameter sections. With these vaporizer tubes, plugs of glass or quartz wool or of a solid, granular separating substance are retained between the throats. The flow of sample liquid through the vaporizer tube is slowed down. The material retained between the throats acts like a packed separating column, through which a pre-separation of the sample is effected. At first, the easily volatile solvent will emerge from the outlet of the vaporizer tube, said solvent being removed by the carrier gas flow before the components of the sample proper are fed by the carrier gas flow to the inlet of the capillary column. Such a packed column as pre-separator delays the passage of the sample. In particular less volatile components are retained.
After sample feeding, the capillary column is subjected to a temperature program. At first, the temperature of the capillary column is low, whereby the migration velocity of all components of the sample is relatively low. This causes, at first, concentration of the sample within a rather short plug at the inlet of the capillary column, a so-called “focusing”. When the temperature of the capillary column is subsequently raised, the various components are transported by the carrier gas flow through the capillary column at different velocities. Such focusing is impeded by packed columns as pre-separators.